This invention concerns a transfer type heat sensitive recording apparatus comprising a mechanism for controlling the tension of an ink donor sheet.
In an apparatus of this type information is recorded by overlaying a heat recording medium supplied with a heat transfer type (heat fusible or sublimating) ink and a recording sheet, and selectively supplying heat from the side of the heat recording medium to thereby transfer the fused or sublimated ink to the recording paper.
FIG. 1 schematically shows the structure of a heat sensitive recording apparatus using, as a heat recording medium, an elongate ink donor sheet having ink on one surface of a thin substrate paper. In the apparatus, the ink donor sheet 2 delivered from a feed roll 1 is pressed by a back roll 4 against a thermal head 8. When a recording paper 9 proceeds between the back roll 4 and the sheet 2 along a path indicated by a dotted line 11 and is passed between the back roll 4 and the thermal head 8, heat transfer recording is conducted. As the recording paper, after recording, proceeds in the direction of a common line tangent to the back roll 4 and a drive roll 5, it is separated from the ink donor sheet 2. The separated recording paper is discharged from the apparatus, whereby heat transfer recording is completed.
The heat sensitive recording apparatus of the type supplying an ink donor sheet from a feed roll has advantages features as compared with an apparatus in which recording is carried out by transporting individually cut ink donor sheets and recording paper while overlaying them, in that (1) no manual separation of the ink donor sheet from the recording paper is necessary and (2) running costs for the ink donor sheet can be reduced. In the apparatus using such a feed roll, however, since the ink donor sheet is transported by itself, creasing may be caused in the sheet thereby rendering it partially impossible to achieve heat transfer recording. Specifically, since the ink donor sheet is made extremely thin to increase the efficiency of the heat transfer, undulations are caused in the surface of the sheet if the tension thereof becomes nonuniform. When the ink donor sheet in this state is pressed against the thermal head, creasing can occur to hinder the ink transfer.
In view of the above, as shown in the conventional device of FIG. 1, a brake member 12 is placed in frictional contact with the outer circumference of the feed roll 1 to provide the ink donor sheet 2 with back tension, and a slip clutch is disposed on the take-up roll 7 so that the ink donor sheet 2 may not be placed under excess tension.
However, this apparatus still involves undesired problems. For example, referring to the brake member 12, since the braking force thereof does not change, the back tension is increased as the diameter for the feed roll is decreased during use. The situation is the same for the slip clutch, and the tension of the ink donor sheet 2 is varied with the change in the diameter of the take-up roll 7. Accordingly, tension of the ink donor sheet cannot be adequately corrected in practice, whereby creasing may still occur in the ink donor sheet 2, and expansion or contraction may result in the recorded images.